In recent years, the development of power supply systems having multiple switching voltage converters has led to improvements in the performance and efficiency of many kinds of electronic equipment.
For example, the demand for ever faster and more complex signal and data processing in diverse fields of application has fuelled the need for new generations of signal processing systems having multiple high-performance integrated circuits (e.g. processors, ASICs and FPGAs), which are characterised by their need for multiple low supply voltages, high levels of current demand and tight supply voltage regulation requirements. These needs are met by multi-converter power supply systems such as the so-called Intermediate Bus Architecture (IBA) power supply, which provides a number of tightly-regulated voltages from an input power source via a two-stage voltage conversion arrangement.
FIG. 1 is a schematic showing an example of a conventional IBA power supply. In the example of FIG. 1, the IBA power system 10 is a two-stage power distribution network comprising a first stage DC/DC converter 20 connected to an input power bus 30, which is typically at a voltage VDCH between 36-75 V, 18-36 V or 18-60 V. The IBC 20 is typically implemented in the efficient form of a switched mode power supply (SMPS), which may be fully regulated or line regulated to convert the input power bus voltage VDCH to a lower intermediate bus voltage VIB on the Intermediate Voltage Bus (IVB) 40. The first stage DC/DC converter 20 is connected via the IVB 40 to the inputs of a number (N) of second stage DC/DC voltage converters 50-1 to 50-N.
In the example of FIG. 1, each of the plurality of second stage DC/DC voltage converters 50-1 to 50-N is a non-isolated buck regulator commonly referred to as a Point-of-Load (POL) regulator. In general, each of the POL regulators may be isolated or non-isolated. However, where isolation is provided by the IBC 20, the POL regulators are preferably all non-isolated. Each POL regulator (k) is an SMPS and delivers a regulated voltage Voutk to its load 60-k by switching a switching element (such as a power MOSFET) in the POL with a switching duty cycle that determines the voltage conversion ratio. In the example of FIG. 1, POL regulators 50-1 and 50-2 deliver power to a common load 60-1 (although more than two POL regulators may deliver power to a common load).
Although the IBC 20 and the POL regulators 50-1 to 50-N are buck regulators in the example of FIG. 1, their topology is not limited to such and may alternatively be Boost, Buck-Boost etc.
In such a power supply system having a plurality of voltage converters 50-1 to 50-N, the switching phases of the converters 50-1 to 50-N may need to be offset relative to one another in order to reduce certain undesirable effects in the system. These undesirable effects include a large ripple voltage in the IVB 40 and high levels of radiated emissions due to synchronized edges of the switching pulses. It should be noted that these problems are not particular to IBA power supplies and arise in many other applications that make use of multiple switching voltage converters, such as current sharing rails and tracking/sequencing groups.
So-called “phase spreading” is one method of mitigating these effects. The simplest way of phase spreading is to allow the voltage converters (in the example of FIG. 1, the POL regulators 50-1 to 50-N) to operate individually, from their own internally generated clock. This randomizes the occurrence of switching pulse edges in time, thereby reducing the likelihood of switching pulses coinciding such that a high peak current is momentarily drawn from the input source that feeds the voltage converters (i.e. the IBC 20 in the example of FIG. 1).
A more controlled and effective way of phase spreading involves controlling the switching of the voltage converters on the basis of a common clock signal, and distributing respective switching pulse edges of the voltage converters through the switching period. In this case, a single clock source is used for all of the converters and each converter has its switching phase offset set to a different value within a period of the switching cycle, Ts. For example in a group of three voltage converters operating with a common duty cycle, phase offsets of Ts/3 may be introduced between the converters. This type of phase spreading effectively reduces the input ripple voltage and also reduces the magnitude and increases the frequency, of radiated emissions.
As the number of voltage converters in such power supply systems increases and the demand for higher currents becomes more common, there is an increasing need for an optimised controlled phase spreading that results in lowest possible input ripple voltage. This is not merely a task of distributing the phase offsets equally throughout a period of the switching cycle because, as will be explained in the following, the contribution from each converter to the input ripple voltage depends on the duty cycle with which the converter operates and the its load current.
Heretofore, it has been usual to manually select the phase offset for each voltage converter when configuring the power supply system prior to use. However, as the number of converters increases, the number of possible combinations of phase offset values increases dramatically. This makes it very difficult to manually find the phase offsets giving the lowest input voltage ripple. Furthermore, known approaches to setting the phase offsets have been much too slow to allow for phase offsets to be optimised “on the fly”, in response to changes in the voltage converters' loads during operation of the power supply system, and thereby maintain a low input voltage ripple.